The present invention relates to compositions and methods for the stripping of tin from a copper metal surface, and more particularly to compositions and methods for stripping tin from a copper metal surface in the course of fabricating a printed circuit board.
In the course of fabricating a printed circuit board, it is commonplace to deposit a layer of tin or tin-lead alloy (solder) on all or selected conductive copper surfaces of the board to serve as an etch resist in the subsequent etching away of copper from other areas of the board. In many of these processes, it also is necessary to later strip the tin or tin-lead from all or selected copper surfaces over which it was deposited. This need arises, for example, where it is necessary to plate nickel or gold directly over copper at certain portions of the board (e.g., contact fingers) or where it is desired to apply a solder mask over certain bare copper surfaces (so-called SMOBC processes). Where tin, rather than tin-lead alloy, was used as the etch resist, stripping of the tin from selected copper surfaces is often a necessary step before provision of those surfaces (e.g., through-holes, surrounding pads, surface mount areas) with a solderability-promoting/solderability-preserving coating, such as a solder coating applied by immersion techniques or hot air solder levelling. Oftentimes, it is simply necessary to strip tin or tin-lead from copper in the course of treating a reject piece in an effort to recover and reuse the underlying copper material.
In addition, when copper metal surfaces are coated with tin or tin-lead alloy, a thin layer or film of copper-tin alloy (intermetallic) typically forms at the layer interface, which film progressively increases in thickness with time. Accordingly, in processes designed to strip away tin or tin-lead alloy to expose underlying copper metal, it is necessary to insure that this coppertin intermetallic also is removed.
The art is replete with teachings directed to compositions and processes for stripping tin or tin-lead alloy from copper surfaces. One class of such compositions includes those based upon hydrogen peroxide and hydrofluoric acid or a fluoride; another class involves those employing nitro-substituted aromatic compounds as a principal ingredient, often in conjunction with an inorganic or organic acid; and another class involves nitric acid-based strippers. See, e.g., U.S. Pat. No. 4,374,744 which discloses a tin or tin-lead stripper made up of an inorganic (hydrochloric, sulfuric, nitric) or organic acid, an oxidizing agent (ferric chloride, hydrogen peroxide) and a nitrogen-containing heterocyclic compound; U.S. Pat. No. 4,424,097, which refers in its background to acid strippers for tin or tin-lead, such as fluoboric or nitric acid strippers, and sulfuric acid strippers; and U.S. Pat. No. 4,713,144, which discloses a nitric acid, ferric nitrate, sulfamic acid composition for stripping tin or tin-lead.
Each of these known tin or tin-lead stripping compositions possesses disadvantageous properties or characteristics. U.S. Pat. Nos. 4,957,653 to Cordani; 4,921,571 to Kukanskis et. al.; 4,944,851 to Cordani et. al.; and 5,017,267 to Cordani, each of which is expressly incorporated herein by reference, contain discussion of these prior art compositions and their disadvantages, and then go on to disclose highly useful and efficient strippers for tin or tin-lead which are based upon solutions of alkane sulfonic acid and ferric nitrate, and which may also include nitric acid, a source of chlorate ion, an inhibitor, and the like.
As reflected in the foregoing and other patents, the general trend in the industry has been to provide a single composition capable of stripping both tin and tin-lead, and which thus can be employed irrespective of whether it is tin or tin-lead which has been used in the printed circuit fabrication process and needs to be stripped. As a consequence, the compositions must contain components effective to oxidize and solubilize both tin and lead ions, and must be sufficiently aggressive to strip tin-lead (and, accordingly, must be properly inhibited against extensive attack of the underlying copper surface). The result typically is a high cost formulation. For those fabricators who make extensive use of tin, and who thus require only a tin stripper, this high cost is a distinct disadvantage.